Antibodies (Abs) contain variable (V) domains at which contacts with antigens are established. The nature and number of these contacts determines the binding specificity of Abs. The human Ab repertoire, defined as the number of Abs with different V domain sequences, is estimated at 1011-1012, each with a potentially different antigen binding specificity.
Recent developments in molecular biology have enabled the development of novel methods whereby medically useful Abs can be isolated from the natural human repertoire and can be improved further by protein engineering techniques. An appreciation of Ab structural organization is helpful in appreciating the scope of the present invention, and a brief review of this aspect follows.
Contacts with antigen epitopes occur mainly at the complementarity determining regions (CDRs) and to a lesser extent the framework regions (FR) of Abs (FIG. 1). Ab diversity is generated by the following processes: (a) inheritance of about 50 germline genes encoding the V domains of each of the two Ab subunits, the light (L) and heavy (H) chains; (b) combinatorial diversity brought about by linkage of different L and H chains within the Ab structure; (c) junctional diversity generated during recombination of the V and joining (J) gene segments of the L chain, and the V, diversity (D) and J gene segments of the H chain; and (d) rapid mutation occurring in the complementarity determining regions over the course of B cell clonal selection, a process entailing binding of the antigen to Abs expressed as components of the B cell receptor (BCR), and resulting in stimulation of division of the B cells expressing BCRs with the highest binding affinity. An additional level of diversity is offered by the use of different constant domains by Abs, that is, the μ, δ, γ, α and ε regions of the H chain and the κ and λ chains of the L chain. Early in the ontogeny of the immune response, Abs contain μ or ι constant regions. Later, isotype switching occurs, and the μ/δ regions are replaced by γ/α/ε in more differentiated Abs.
The purpose of Abs made by the healthy immune system is to protect against pathogens. Such Ab responses are usually mounted upon exposure to foreign antigens, e.g., in microbial infection. However, in some cases the human immune system makes Abs that react with some foreign antigens even without prior exposure to the foreign antigen. For example, Abs to viral antigens are found in patients with autoimmune disease without evidence of infection, e.g., e.g., Abs to human T lymphotropic virus (HTLV-1) in lupus patients and Abs to certain retroviral antigens in multiple sclerosis. In the case of HIV-1, uninfected patients with lupus or with mixed connective tissue disease are known to express Abs to the envelope protein of the virus (1,2).
In many cases, the protective immune responses against microbes in infected subjects are insufficient to control infection. This occurs, for example, in humans who are immunodeficient. Furthermore, many microbes use immune subversion mechanisms to establish infection, with the result that infected individuals produce Abs that do not protect adequately in the spread of infection. Monoclonal Abs to microbial proteins that can be administered to such individuals as passive immunotherapeutic reagents could provide protection against infection.
In the present invention, patients with autoimmune disease are identified as a source of such monoclonal Abs. The unique properties of the anti-microbial Abs from patients with autoimmune disease are likely due, in part to their unique genesis. These Abs are produced on account of enhanced autoreactive immunological responses. The autoantigens serving as targets of immune responses include polypeptides derived from endogenous retroviral (ERV) sequences found in the heritable genome of higher organisms, some of which are homologous in sequence to modem-day microbial antigens. Between 1% and 8% of the human genome is composed of ERVs (HERVs), which are thought to have been acquired gradually over the course of evolution. The human genome contains about 3 billion bases, of which about 3% are expressed. There is extensive evidence that retroviral sequences are expressed in autoimmune diseases, and this process has been proposed as a mechanism underlying pathogenic autoimmune responses (reviewed in 3,4).